Measuring the Night Sky (Coordinates and Field of View)

The Celestial Sphere

Like the surface of the Earth, the night sky has a series of coordinates that help us locate objects that appear in the sky. The coordinates are laid out on an imaginary sphere (the Celestial Sphere) that represents the sky. The Celestial Equator cuts the sphere in half (north vs. south). There are also the North and South Celestial Poles.

One can also “draw” imaginary vertical lines from pole to pole. These are called meridians and also cut the celestial sphere in half (east vs. west).

Note that the ecliptic (the path of the Sun) is not the same as the celestial equator. The ecliptic runs north of the celestial equator for part of the year and south of it for the other part of the year. It crosses the celestial equator at the equinoxes.

Celestial Sphere Credit: NASA

The Earth is divided into latitude and longitude. The celestial sphere uses similar coordinates, but they are referred to as right ascension and declination. The stars and deep sky objects all have fixed coordinates. The planets, Moon, and Sun, however, move around and their position in the sky and their coordinates vary.

  • Right Ascension (RA) is similar to longitude. It is measured in hours, minutes, and seconds around the horizontal circumference of the sky starting from an imaginary vertical line called the prime meridian. This is the meridian that crosses the point where the ecliptic (apparent path of the Sun) intersects the celestial equator marking the beginning of spring (the Vernal Equinox).
  • Declination (DEC) is similar to latitude. It is measured in positive degrees starting from zero at the celestial equator to +90o at the north celestial pole, and in negative degrees to -90o at the south celestial pole.

A Few References

Horizontal Coordinates (Altitude and Azimuth)

When we go out at night and set up our telescope, we typically don’t think about celestial coordinates. We look up and we look around. Unless we have an equatorial mount, which is alighted with the celestial sphere, our telescope moves horizontally (azimuth) and vertically (altitude). In this case, we follow the visible sky. We can describe the position of an object in the night sky at a specific point in time with these coordinates. However, as the Earth rotates, an object’s position changes.

  • Azimuth (AZ) is the horizontal direction in which we’re looking. North is at 0o, East at 90o, South at 180o, and West at 270o.
  • Altitude (ALT) is the vertical height in which we’re looking. We can identify the zenith (at 90o), which is the point in the sky directly above us, and the horizon (at 0o).

The local meridian for any observer, is the meridian that goes from north to south and passes through the zenith.

A Few References

Degrees and Field of View

When we look up at the sky, we typically focus on only a small portion. We describe this as our field of view. The field of view determines the width of the sky you can see and is typically measured in degrees. We can also describe the apparent size of an object in the sky (e.g., the Sun or Moon) and the apparent distance between objects in degrees. Degrees are often broken down into smaller increments: arc minutes (60’ in a degree) and arc seconds (60” in an arc minute). The Moon and Sun are both approximately 1/2 degree across and the Big Dipper stretches 25 degrees across the sky.

The sky is divided into 360o.  
At any time during the night, you can see 180o from the southern horizon to the northern horizon (or from east to west).
The point in the sky directly above you is 90o from the horizon.

You can use your hand at arms length to estimate the width of a portion of the sky (see the image to the right). 

A Few References